learning-haskell/19-applying-structure/19-general-remarks.md

Chapter 19 - General Remarks

Concatenating key configurations

In the example with XMonad, the following code is shown:

main = do
  xmonad def { keys =
    \c -> fromList [
      ((0, xK_F6),
        lowerVolume 4 >> return ()),
      ((0, xK_F7),
        raiseVolume 4 >> return ())
    ] `mappend` keys defaultConfig c
  }

The monoid is quite clear from here, but I did not understand the def { keys = ... } part. After looking at the tutorial from XMonad for configuring: XMonad Doc Configuring, I discovered that this was using record update syntax. This allows you to change fields of a record type, e.g.

Prelude> data User = User { name :: String, age :: Integer } deriving Show
Prelude> let a = User { name = "John", age = 33 }
Prelude> let b = b { name = "Jane" }
Prelude> a
User { name = "John", age = 33 }
Prelude> b
User { name = "Jane", age = 33 }

19.3 - lifting over web monad

I don't quite understand how this works. I believe it should be a single fmap. I'm even more convinced after looking at the snap library:

getRequest :: MonadSnap m => m Request
getHeader :: HasHeaders a => CI ByteString -> a -> Maybe ByteString

UserAgent is probably defined as type UserAgent = ByteString and with OverloadedString`, we do have:

userAgent' :: Request -> Maybe UserAgent
userAgent' req = getHeader "user-agent" req

Which is correct. In order to apply the userAgent' function to the getRequest, we only to need lift over one structure, not two as getRequest only has one structure.